Active material, battery, and methods for producing these

ABSTRACT

A main object of the present disclosure is to provide an active material wherein a volume variation due to charge/discharge is small. The present disclosure achieves the object by providing an active material comprising at least Si and Al, including a silicon clathrate type crystal phase, and a proportion of the Al to a total of the Si and the Al is 0.1 atm % or more and 1 atm % or less.

TECHNICAL FIELD

The present disclosure relates to an active material, a battery, andmethods for producing these.

BACKGROUND ART

In recent years, development of batteries has been actively undergone.For example, in automobile industries, the development of a battery tobe utilized for an electronic vehicle or a hybrid vehicle has beenadvanced. Also, as an active material used in the battery, Si has beenknown.

For example, Patent Literature 1 discloses that a silicon clathrate maybe calculatory used as an anode active material of a lithium ionbattery. Specifically, Si₄₆ and Si₃₄ are disclosed as the siliconclathrate.

Also, Patent Literature 2 discloses an electrode active materialcomprising a clathrate compound including a crystal lattice, and a guestsubstance included in the crystal lattice. Specifically, PatentLiterature 2 discloses that the guest substance includes at least onekind selected from the group consisting of Ba, Ca, and Li; the crystallattice includes at least one kind selected from the group consisting ofGa, Al, In, Ag, Au, Cu, Ni, and Co, and at least one kind selected fromthe group consisting of Si and Sn.

Also, Non-patent Literature 1 discloses a ternary silicon clathrateincluding I type crystal phase.

CITATION LIST Patent Literatures

-   Patent Literature 1: US Patent Application Laid-Open No.    2012/0021283 Specification-   Patent Literature 2: WO2014/050100

Non-Patent Literature

-   Non-patent Literature 1: Tiago F. T. Cerqueira et al., “Prediction    and Synthesis of a Non-Zintl Silicon Clathrate”, Chem. Mater. 2016,    28, 3711-3717

SUMMARY OF DISCLOSURE Technical Problem

The theoretical capacity of Si is large, which is advantageous for thehigher energy density of a battery. On the other hand, the volumevariation of Si due to charge/discharge is large.

The present disclosure has been made in view of the above circumstances,and a main object of the present disclosure is to provide an activematerial wherein a volume variation due to charge/discharge is small.

Solution to Problem

In order to achieve the object, the present disclosure provides anactive material comprising at least Si and Al, including a siliconclathrate type crystal phase, and a proportion of the Al to a total ofthe Si and the Al is 0.1 atm % or more and 1 atm % or less.

According to the present disclosure, since the active material includesa silicon clathrate type crystal phase, the volume variation due tocharge/discharge may be diminished. Further, since the active materialin the present disclosure includes Al in addition to Si, the volumevariation due to charge/discharge may be diminished.

In the disclosure, the active material may include a void inside aprimary particle.

In the disclosure, a void ratio of the void may be 2% or more and 15% orless.

In the disclosure, the active material may include a silicon clathrateII type crystal phase as the silicon clathrate type crystal phase.

Also, the present disclosure provides a battery comprising a cathodelayer, an anode layer, and an electrolyte layer formed between thecathode layer and the anode layer, and the anode layer includes theabove described active material.

According to the present disclosure, a battery wherein the volumevariation due to charge/discharge is small may be obtained by using theabove described active material.

Also, the present disclosure provides a method for producing an activematerial, the method comprising: a preparing step of preparing a Zintlcompound including Na, Si, and Al, including at least a Zintl phase, anda proportion of the Al to a total of the Si and the Al is less than 10atm %, a Na removing step of removing the Na from the Zintl compound,and forming an intermediate including a silicon clathrate type crystalphase, and an Al removing step of removing the Al from the intermediate.

According to the present disclosure, an active material wherein thevolume variation due to charge/discharge is small may be obtained byusing the Zintl compound including Al in addition to Na and Si.

In the disclosure, the preparing step may be a step of preparing theZintl compound by carrying out a heat treatment to a raw materialmixture including a Na source, a Si source and an Al source.

Also, the present disclosure provides a method for producing a battery,the method comprising: an active material producing step of producing anactive material by the above described method for producing an activematerial, and an anode layer forming step of forming an anode layer byusing the active material.

According to the present disclosure, a battery wherein the volumevariation due to charge/discharge is small may be obtained by using theabove described active material.

Advantageous Effects of Disclosure

The present disclosure exhibits effects such that an active materialwherein a volume variation due to charge/discharge is small, may beobtained.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are schematic perspective views illustrating a Si crystalphase.

FIG. 2 is a schematic cross-sectional view illustrating an example ofthe battery in the present disclosure.

FIG. 3 is a flow chart illustrating an example of the method forproducing an active material in the present disclosure.

FIGS. 4A and 4B are the results of an XRD measurement of theintermediate and the active material obtained in Example 2.

FIG. 5 is the result of a SEM observation of the active materialobtained in Example 2.

FIGS. 6A to 6E are the results of a SEM observation (cross-section) ofthe active materials obtained in Examples 1 to 4 and Comparative Example3.

DESCRIPTION OF EMBODIMENTS

The active material, the battery, and the methods for producing these inthe present disclosure are hereinafter described in detail.

A. Active Material

The active material in the present disclosure comprises at least Si andAl, includes a silicon clathrate type crystal phase, and a proportion ofthe Al to a total of the Si and the Al is 0.1 atm % or more and 1 atm %or less.

The active material in the present disclosure comprises a siliconclathrate type crystal phase. The framework atom of the siliconclathrate type crystal phase includes a basket-like structure (cage),and a metal ion such as Li ion may enter therein. Since the expansionquantity is small even when the metal ion enters the cage, the volumevariation due to charge/discharge is small. Examples of the siliconclathrate type crystal phase may include silicon clathrate I type and IItype crystal phase.

In the silicon clathrate I type and II type crystal phase, as shown inFIG. 1A and FIG. 1B, a polyhedron including a pentagon or a hexagon isformed with a plural of Si elements. The polyhedron has a space withinthereof that is capable of including a metal ion such as a Li ion. By ametal ion being intercalated into this space, the volume variation dueto charge/discharge may be suppressed. Also, since the silicon clathrateI type and II type crystal phase includes a space within thereof that iscapable of including a metal ion, there is an advantage that the crystalstructure is likely to be maintained even charged and dischargedrepeatedly. Also, particularly in an all solid state battery, a highconfining pressure is generally needed to be applied in order tosuppress the volume variation due to charge/discharge. However, theconfining pressure may be reduced by using the active material in thepresent disclosure; as the result, an enlargement of a confining jig maybe suppressed. Meanwhile, a usual Si particle includes a diamond typecrystal phase. In the diamond type crystal phase, as shown in FIG. 1C, atetrahedron is formed with a plural of Si elements. Since thetetrahedron does not have a space within thereof that is capable ofincluding a metal ion such as a Li ion, the volume variation due tocharge/discharge is large.

As described above, according to the present disclosure, since theactive material includes a silicon clathrate type crystal phase, thevolume variation due to charge/discharge may be diminished. Further,since the active material in the present disclosure includes Al inaddition to Si, the volume variation due to charge/discharge may bediminished. The reason therefor is presumed that the cage get larger bya part of Si composing the cage of the silicon clathrate type crystalphase being substituted with Al, a different element. Also, since thecage get larger, the resistance when a metal ion is conducted islowered. Also, as described later, the active material in the presentdisclosure preferably includes a void inside a primary particle, and inthis case, the volume variation due to charge/discharge may further besuppressed since the void also contribute to the suppression of thevolume variation, in addition to the silicon clathrate type crystalphase.

The active material in the present disclosure includes at least Si andAl. The active material may only include Si and Al as the metal element,and may include other metal element. Examples of the other metal elementmay include Na. Also the active material may include only Si, Al and Naas the metal element. The proportion of the total of Si and Al to allthe metal elements included in the active material is, for example, 60atm % or more, may be 70 atm % or more, and may be 80 atm % or more.

In the active material, the proportion of Al to the total of Si and Alis usually 0.1 atm % or more, and may be 0.2 atm % or more. Meanwhile,the proportion of Al to the total of Si and Al is usually 1 atm % orless.

The composition of the active material in the present disclosure is notparticularly limited, and is preferably a composition represented byNa_(x)(Si, Al)₁₃₆ (0≤x≤20). The “x” may be 0, and may be more than 0.Meanwhile the “x” may be 10 or less, and may be 5 or less. Thecomposition of the active material may be determined by, for example,EDX, XRD, XRF, ICP, and an atomic absorption spectrometry.

The active material in the present disclosure includes a siliconclathrate type crystal phase. The active material preferably includesthe silicon clathrate type as a main phase. “Including the siliconclathrate type as a main phase” indicates that, among the peaks observedin X-ray diffraction measurement, a peak belonging to the siliconclathrate type crystal phase is the peak with the strongest diffractionintensity. The definition relating “main phase” is similar for othercrystal phase. Also, the silicon clathrate type crystal phase includesat least Si, and may further include Al. That is, a part of Si composingthe silicon clathrate type crystal phase may be substituted with Al.

Examples of the silicon clathrate type crystal phase may include asilicon clathrate I type crystal phase and a silicon clathrate II typecrystal phase. The active material in the present disclosure may includethe silicon clathrate I type crystal phase as a main phase, and mayinclude the silicon clathrate II type crystal phase as a main phase, andthe latter is preferable. The reason therefor is to further suppress thevolume variation due to charge/discharge.

Also, the active material in the present disclosure may (i) include thesilicon clathrate I type crystal phase and no silicon clathrate II typecrystal phase, (ii) include no silicon clathrate I type crystal phaseand include the silicon clathrate II type crystal phase, and (iii)include the silicon clathrate I type crystal phase and the siliconclathrate II type crystal phase. “Including no crystal phase” may beconfirmed by the peak of the crystal phase not being observed in X-raydiffraction measurement. The definition relating “including no crystalphase” is similar for other crystal phase.

The silicon clathrate I type crystal phase usually belongs to the spacegroup (Pm-3n). The silicon clathrate I type crystal phase has a typicalpeak at a position of 2θ=19.44°, 21.32°, 30.33°, 31.60°, 32.82°, 36.29°,52.39°, and 55.49° in X-ray diffraction measurement using a CuKα ray.These peak positions may be shifted respectively in a range of ±0.50°,may be shifted in a range of ±0.30°, and may be shifted in a range of±0.10°.

The silicon clathrate II type crystal phase usually belongs to the spacegroup (Fd-3m). The silicon clathrate II type crystal phase has a typicalpeak at a position of 28=20.09°, 21.00°, 26.51°, 31.72°, 36.26°, and53.01° in X-ray diffraction measurement using a CuKα ray. These peakpositions may be shifted respectively in a range of ±0.50°, may beshifted in a range of ±0.30°, and may be shifted in a range of ±0.10°.

The lattice constant of the silicon clathrate II type crystal phase is,for example, 14.702 Å or more, and may be 14.706 Å or more. Meanwhile,the lattice constant of the silicon clathrate II type crystal phase is,for example, 14.717 Å or less. The lattice constant may be determined bycarrying out XRD measurement to the active material, and analyzing theobtained XRD chart with Rietveld method.

Also, the active material in the present disclosure may or may notinclude diamond type Si crystal phase. The diamond type Si crystal phasehas a typical peak at a position of 2θ=28.44°, 47.31°, 56.10°, 69.17°,and 76.37°, in an X-ray diffraction measurement using a CuKα ray. Thesepeak positions may be shifted respectively in a range of ±0.50°, may beshifted in a range of ±0.30°, and may be shifted in a range of ±0.10°.

The peak intensity of peak “A” at a position of 28=31.72°±0.50° in thesilicon clathrate II type crystal phase is regarded as I_(A), and thepeak intensity of peak “B” at a position of 2θ=28.44°±0.50° in thediamond type Si crystal phase is regarded as I_(B). I₃/I_(A) is, forexample, less than 1, may be 0.5 or less, and may be 0.3 or less.

Also, the active material in the present disclosure preferably has nopeak deriving from the bi-product described later. The details for thebi-product will be described in “C. Method for producing activematerial” later.

Examples of the shape of the active material in the present disclosuremay include a granular shape. The active material may be a primaryparticle, and may be a secondary particle wherein the primary particlesare agglutinated. In either case, the active material preferablyincludes a void inside the primary particle. The void ratio of the voidis, for example, 2% or more, may be 5% or more, and may be 10% or more.Also, the void ratio is, for example, 40% or less, may be 20% or less,and may be 15% or less.

The void ratio may be determined by, for example, the followingprocedure. First, a cross-section of an electrode layer including anactive material is obtained by conducting an ion milling processthereto. Then, the cross-section is observed with a SEM (scanningelectron microscope), and a photograph of the particle is taken. In theobtained photograph, the silicon part and the void part are rigidlydistinguished and digitalize with an image analyzing software. The areasof the silicon part and the void part are determined, and the void ratio(%) is calculated from the below described formula.Void ratio (%)=100×(void part area)/((silicon part area)+(void partarea))

The specific image analyzing and the calculation of the void ratio maybe conducted as described below. As the image analyzing software, forexample, Fiji ImageJ bundled with Java 1.8.0172 (hereinafter, Fiji) isused. The image is colorized into an RGB color image by combining asecondary electron image and a reflection electron image in the samefield of view. Then, in order to eliminate the noise of each pixel, theobtained RGB image is blurred with the function of Fiji “Median (filtersize=2)”. Next, with the function of Fiji “Weka Machine Learning”, aplurality of arbitrary regions in the noise eliminated image arespecified into the silicon part or the void part respectively by aperson, and teaching data wherein the silicon part and the void part arerigidly distinguished, are formed. Then, based on the formed teachingdata, the silicon part and the void part are discriminated with amachine in Fiji, and the area ratio of the silicon part and the voidpart is calculated.

In relation to the colorizing into the RGB color image, since both ofthe secondary electron image and the reflection electron image aredisplayed in a grayscale, the brightness “x” of each pixel in thesecondary electron image is assigned to Red value, and the brightness“y” in the reflection electron image is similarly assigned to Greenvalue, for example. Thereby, each pixel are colorized into an RGB colorimage as, for example, R=x, G=y, B=(x+y)/2.

The detailed conditions in “Weka Machine Learning” described above willbe hereinafter described. As training features (numerical features of animage to be focused by a machine when forming teaching data in a machinelearning), Gaussian blur, Hessian, Membrane projections, Mean, Maximum,Anisotropic diffusion, Sobel filter, Difference of gaussians, Variance,Minimum, Median are selected. Also, for other parameters, Membranethickness is set to 3, Membrane patch size is set to 19, Minimum sigmais set to 1.0, and Maximum sigma is set to 16.0.

The average particle size of the primary particle is, for example, 50 nmor more, may be 100 nm or more, and may be 150 nm or more. Meanwhile,the average particle size of the primary particle is, for example, 3000nm or less, may be 1500 nm or less, and may be 1000 nm or less. Also,the average particle size of the secondary particle is, for example, 1μm or more, may be 2 μm or more, and may be 5 μm or more. Meanwhile, theaverage particle size of the secondary particle is, for example, 60 μmor less, and may be 40 μm or less. Incidentally, the average particlesize may be determined by observation with a SEM, for example. Thenumber of the sample is preferably large; for example, 20 or more, maybe 50 or more, and may be 100 or more.

The active material in the present disclosure is usually used for abattery. The active material in the present disclosure may be an anodeactive material, may be a cathode active material, and the former ispreferable. In the present disclosure, an electrode layer (anode layeror cathode layer) including the above described active material, and abattery including the electrode layer may be provided. Examples of themethod for producing an active material may include the method forproducing described in “C. Method for producing active material” later.

B. Battery

FIG. 2 is a schematic cross-sectional view illustrating an example ofthe battery in the present disclosure. Battery 10 shown in FIG. 2comprises cathode layer 1, anode layer 2, electrolyte layer 3 formedbetween cathode layer 1 and anode layer 2, cathode current collector 4for collecting currents of cathode layer 1, and anode current collector5 for collecting currents of anode layer 2. In the present disclosure,anode layer 2 includes the active material described in “A. Activematerial” above.

According to the present disclosure, a battery wherein the volumevariation due to charge/discharge is small, may be obtained by using theabove described active material.

1. Anode Layer

The anode layer is a layer including at least an anode active material.The anode active material may be in the same contents as those describedin “A. Active material” above; thus, the description herein is omitted.The proportion of the anode active material in the anode layer is, forexample, 20 weight % or more, may be 30 weight % or more, and may be 40weight % or more. When the proportion of the anode active material istoo low, a sufficient energy density may not be obtained. Meanwhile, theproportion of the anode active material is, for example, 80 weight % orless, may be 70 weight % or less, and may be 60 weight % or less. Whenthe proportion of the anode active material is too high, an ionconductivity and an electron conductivity in the anode layer may bereduced, relatively.

The anode layer may include at least one of an electrolyte, a conductivematerial, and a binder as required. Examples of the electrolyte mayinclude the electrolyte which will be described in “3. Electrolytelayer” later. Examples of the conductive material may include a carbonmaterial, a metal particle, and a conductive polymer. Examples of thecarbon material may include particulate carbon materials such asacetylene black (AB) and Ketjen black (KB); and fibrous carbon materialssuch as carbon fiber, carbon nanotube (CNT), and carbon nanofiber (CNF).Also, examples of the binder may include rubber-based binders andfluorine-based binders.

The thickness of the anode layer is, for example, 0.1 μm or more and1000 μm or less. The anode layer in the present disclosure is usuallyused for a battery.

2. Cathode Layer

The cathode layer is a layer including at least a cathode activematerial. Also, the cathode layer may include at least one of anelectrolyte, a conductive material, and a binder, as necessary.

Examples of the cathode active material may include an oxide activematerial. Examples of the oxide active material may include rock saltbed type active materials such as LiCoO₂, LiMnO₂, LiNiO₂, LiVO₂,LiNi_(1/3)CO_(1/3)Mn_(1/3)O₂; spinel type active materials such asLiMn₂O₄, Li₄Ti₅O₁₂, and Li(Ni_(0.5)Mn_(1.5))O₄; and olivine type activematerials such as LiFePO₄, LiMnPO₄, LiNiPO₄, and LiCoPO₄.

A coating layer including a Li ion conductive oxide may be formed on thesurface of the oxide active material. The reason therefor is to suppressthe reaction of the oxide active material with the solid electrolyte(particularly sulfide solid electrolyte). Examples of the Li ionconductive oxide may include LiNbO₃. The thickness of the coating layeris, for example, 1 nm or more and 30 nm or less. Also, Li₂S may be used,for example, as the cathode active material.

Examples of the shape of the cathode active material may include agranular shape. The average particle size (D₅₀) of the cathode activematerial is not particularly limited; is, for example, 10 nm or more,and may be 100 nm or more. Meanwhile, the average particle size (D₅₀) ofthe cathode active material is, for example, 50 μm or less, and may be20 μm or less. The average particle size (D₅₀) may be calculated fromthe measurement by, for example, a laser diffraction type particle sizedistribution meter, and a scanning electron microscope (SEM).

The electrolyte, the conductive material and the binder used for thecathode layer may be in the same contents as those described in “1.Anode layer” above; thus, the description herein is omitted. Thethickness of the cathode layer is, for example, 0.1 μm or more and 1000μm or less.

3. Electrolyte Layer

The electrolyte layer is a layer formed between the cathode layer andthe anode layer, and includes at least an electrolyte. The electrolytemay be a solid electrolyte, and may be an electrolyte solution (liquidelectrolyte).

Examples of the solid electrolyte may include inorganic solidelectrolytes such as sulfide solid electrolyte, oxide solid electrolyte,nitride solid electrolyte, and halide solid electrolyte; and organicpolymer electrolytes such as polymer electrolyte. Examples of thesulfide solid electrolyte may include solid electrolyte including Li, X(X is at least one kind of P, As, Sb, Si, Ge, Sn, B, Al, Ga, and In) andS. Also, the sulfide solid electrolyte may further include at leasteither one of O and halogen. Examples of the halogen may include F, Cl,Br, and I. The sulfide solid electrolyte may be a glass (amorphous), andmay be a glass ceramic. Examples of the sulfide solid electrolyte mayinclude Li₂S—P₂S₅, LiI—Li₂S—P₂S₅, LiI—LiBr—Li₂S—P₂S₅, Li₂S—SiS₂,Li₂S—GeS₂, and Li₂S—P₂S₅—GeS₂.

The liquid electrolyte preferably includes a supporting salt and asolvent. Examples of the supporting salt (lithium salt) of the liquidelectrolyte having lithium ion conductivity may include inorganiclithium salts such as LiPF₆, LiBF₄, LiClO₄, and LiAsF₆; and organiclithium salts such as LiCF₃SO₃, LiN (CF₃SO₂)₂, LiN (C₂F₅SO₂)₂,LiN(FSO₂)₂, and LiC(CF₃SO₂)₃. Examples of the solvent used for theliquid electrolyte may include cyclic esters (cyclic carbonates) such asethylene carbonate (EC), propylene carbonate (PC), and butylenecarbonate (BC); and chain esters (chain carbonates) such as dimethylcarbonate (DMC), diethyl carbonate (DEC), and ethylmethyl carbonate(EMC). The liquid electrolyte preferably includes two kinds or more ofthe solvents.

The thickness of the electrolyte layer is, for example, 0.1 μm or moreand 1000 μm or less.

4. Other Constitutions

The battery in the present disclosure preferably comprises a cathodecurrent collector for collecting currents of the cathode layer and ananode current collector for collecting currents of the anode layer.Examples of the materials for the cathode current collector may includeSUS, aluminum, nickel, iron, titanium, and carbon. Meanwhile, examplesof the materials for the anode current collector may include SUS,copper, nickel, and carbon.

The battery in the present disclosure may further include a confiningjig that applies a confining pressure along the thickness direction, tothe cathode layer, the electrolyte layer and the anode layer.Particularly, when the electrolyte layer is a solid electrolyte layer,the confining pressure is preferably applied in order to form afavorable ion conductive path and an electron conductive path. Theconfining pressure is, for example, 0.1 MPa or more, may be 1 MPa ormore, and may be 5 MPa or more. Meanwhile, the confining pressure is,for example, 100 MPa or less, may be 50 MPa or less, and may be 20 MPaor less.

5. Battery

The kind of the battery in the present disclosure is not particularlylimited; and is typically a lithium ion battery. Also, the battery inthe present disclosure may be a liquid battery in which a liquidelectrolyte is included as the electrolyte layer, and may be an allsolid state battery in which a solid electrolyte layer is included asthe electrolyte layer. Also, the battery in the present disclosure maybe a primary battery and may be a secondary battery; above all,preferably the secondary battery so as to be repeatedly charged anddischarged, and be useful as a car-mounted battery, for example.

The battery in the present disclosure may be a single cell battery andmay be a stacked battery. The stacked battery may be a monopolar typestacked battery (a stacked battery connected in parallel), and may be abipolar type stacked battery (a stacked battery connected in series).Examples of the shape of the battery may include a coin shape, alaminate shape, a cylindrical shape, and a square shape.

C. Method for Producing Active Material

FIG. 3 is a flow chart illustrating an example of the method forproducing an active material in the present disclosure. At first, in themethod for producing shown in FIG. 3 , a Zintl compound including Na,Si, and Al, including at least a Zintl phase, and a proportion of Al toa total of Si and Al is less than 10 atm %, is prepared (preparingstep). Next, Na is removed from the Zintl compound, and an intermediateincluding a silicon clathrate type crystal phase is formed (Na removingstep). Then, Al is removed from the intermediate (Al removing step).Thereby, an active material including a silicon clathrate type crystalphase, and also including a void inside a primary particle may beobtained.

According to the present disclosure, an active material wherein thevolume variation due to charge/discharge is small may be obtained byusing the Zintl compound including Al in addition to Na and Si.

1. Preparing Step

The preparing step in the present disclosure is a step of preparing aZintl compound including Na, Si, and Al, including at least a Zintlphase, and a proportion of the Al to a total of the Si and the Al isless than 10 atm %.

The Zintl compound includes at least Na, Si, and Al. As the metalelement, the Zintl compound may include only Na, Si and Al, and mayinclude other metal element. The proportion of the total of Na, Si andAl to all the metal elements included in the Zintl compound is, forexample, 70 atm % or more, may be 80 atm % or more, and may be 90 atm %or more.

In the Zintl compound, the proportion of the Al to a total of the Si andthe Al is, for example, 0.1 atm % or more, may be 0.5 atm % or more, andmay be 1 atm % or more. Meanwhile, the proportion of the Al to a totalof the Si and the Al is usually less than 10 atm %, and may be 8 atm %or less. The composition of the Zintl compound is not particularlylimited; and a composition represented by Na_(z)(Si, Al)₁₃₆, (121≤z≤151)is preferable. The “z” may be 126 or more, and may be 131 or more.Meanwhile, the “z” may be 141 or less.

The Zintl compound includes a Zintl phase. The Zintl phase has a typicalpeak at a position of 28=16.10°, 16.56°, 17.64°, 20.16°, 27.96°, 33.60°,35.68°, 40.22°, and 41.14° in X-ray diffraction measurement using a CuKαray. These peak positions may be shifted respectively in a range of±0.50°, and may be shifted in a range of ±0.30°. The Zintl compoundpreferably includes the Zintl phase as a main phase. The Zintl compoundmay or may not include a silicon clathrate I type crystal phase. Also,the Zintl compound may or may not include a silicon clathrate II typecrystal phase.

The intermediate may include a Zintl phase including Na, Si and Al, anda by-product including at least Al. The Zintl phase preferably includesNa, Si and Al. A typical Zintl phase is composed of Na and Si; however,it is presumed that, when a part of Si thereof is substituted with Al, adesired silicon clathrate type crystal phase (a silicon clathrate typecrystal phase wherein a part of Si is substituted with Al) is likely tobe obtained.

The by-product includes at least Al. The by-product preferably has apeak at a position of 2θ=34.8°, and 36.0° in X-ray diffractionmeasurement using a CuKα ray. These peak positions may be shiftedrespectively in a range of ±0.3°, and may be shifted in a range of±0.1°. In addition to Al, the by-product may include at least either oneof Na and O.

The Zintl compound may be obtained by carrying out a heat treatment to araw material mixture including a Na source, a Si source, and Al source.Examples of the Na source may include a simple substance of Na, and analloy including Na as a main component. Examples of the Si source mayinclude a simple substance of Si, and an alloy including Si as a maincomponent. Examples of the Al source may include a simple substance ofAl, an alloy including Al as a main component, and an Al oxide. Examplesof the Al oxide may include Al₂O₃.

The proportion between Na and the total of Si and Al in the raw materialmixture is not particularly limited; to 1 mol part of the total of Siand Al, Na is, for example, 0.8 mol parts or more, and may be 1 mol partor more. Meanwhile, to 1 mol part of the total of Si and Al, Na is, forexample, 1.5 mol parts or less, and may be 1.3 mol parts or less. Also,the proportion of Al to the total of Si and Al in the raw materialmixture is, for example, 0.1 atm % or more, may be 0.5 atm % or more,and may be 1 atm % or more. Meanwhile, the proportion of Al to the totalof Si and Al in the raw material mixture is, for example, less than 10atm %, and may be 8 atm % or less.

The heat treatment temperature is, for example, 500° C. or more and1000° C. or less. Also, the heat treating time is, for example, 1 houror more and 50 hours or less. Particularly, it is preferable to conductthe heat treatment under the conditions of approximately 700° C. (suchas 650° C. or more and 750° C. or less) and approximately 20 hours (suchas 15 hours or more and 25 hours or less).

2. Na Removing Step

The Na removing step in the present disclosure is a step of removing theNa from the Zintl compound, and forming an intermediate including asilicon clathrate type crystal phase.

Examples of the method for removing Na from the Zintl compound mayinclude a heat treatment. The heat treatment temperature is, forexample, 280° C. or more, and may be 300° C. or more. Meanwhile, theheat treatment temperature is, for example, 500° C. or less. The heattreating time is, for example, 1 hour or more and 50 hours or less. Theheat treatment may be conducted under ambient pressure atmosphere, andmay be conducted under reduced pressure atmosphere. In the latter case,the pressure at the time of heat treatment is, for example, 10 Pa orless, may be 1 Pa or less, and may be 0.1 Pa or less. Also, the heattreatment may be conducted under inert atmosphere such as an Aratmosphere.

The intermediate includes a silicon clathrate type crystal phase. Theintermediate preferably includes the silicon clathrate type crystalphase as a main phase. The intermediate may include a silicon clathrateI type crystal phase as a main phase, and may include a siliconclathrate II type crystal phase as a main phase, and the latter ispreferable. The reason therefor is to further suppress the volumevariation due to charge/discharge.

Also, the intermediate may (i) include the silicon clathrate I typecrystal phase and no silicon clathrate II type crystal phase, (ii)include no silicon clathrate I type crystal phase and include thesilicon clathrate II type crystal phase, and (iii) include the siliconclathrate I type crystal phase and the silicon clathrate II type crystalphase. The composition of the intermediate is not particularly limited;and a composition represented by Na_(y)(Si, Al)₁₃₆ (0≤y≤24) ispreferable. The “y” may be 0, and may be more than 0. Meanwhile, the “y”may be 20 or less, and may be 10 or less.

3. Al Removing Step

The Al removing step in the present disclosure is a step of removing theAl from the intermediate. The phrase “remove Al” includes a case where acompound including Al (such as above described by-product) is removed.By removing Al, an active material including a void inside a primaryparticle may be obtained.

Examples of a method for removing Al from the intermediate may includean acid treatment. The acid treatment is, for example, a treatmentwherein the intermediate is brought into contact with an acid solution.The acid solution includes, for example, an acid and a solvent. Examplesof the acid used for the acid solution may include hydrochloric acid,sulfuric acid, acetic acid, formic acid, propionic acid, oxalic acid,and hydrofluoric acid. The solvent used for the acid solution may bewater, and may be an organic solvent. Also, the acid concentration(normality) in the acid solution is not particularly limited, and is,for example, 0.5 N or more and 3 N or less. Incidentally, instead of theacid solution, an acid itself, not diluted with a solvent, may be used.

4. Active Material

The active material obtained by each of the above described stepincludes the silicon clathrate type crystal phase. Also, the activematerial includes at least Si and Al, and the proportion of the Al to atotal of the Si and the Al is preferably in a predetermined range.Further, the active material preferably includes a void inside a primaryparticle. For the preferable embodiment of the active material, thecontents described in “A. Active material” above may be appropriatelyreferred.

D. Method for producing battery.

The present disclosure provides a method for producing a battery, themethod comprising: an active material producing step of producing anactive material by the above described method for producing an activematerial, and an anode layer forming step of forming an anode layerusing the active material.

According to the present disclosure, by using the above described activematerial, a battery wherein a volume variation due to charge/dischargeis small, may be obtained. Incidentally, the active material producingstep may be in the same contents as those described in “C. Method forproducing active material” above.

In the anode layer forming step, an anode layer is formed using theactive material. The method for forming an anode layer is notparticularly limited, and a known method may be adopted. An anode layerformed on an anode current collector may be obtained by, for example,coating the anode current collector with a slurry including at least anactive material, and drying. For the preferable embodiment of theobtained anode layer, the contents described in “B. Battery, 1. Anodelayer” above may be appropriately referred.

The method for forming a battery is not particularly limited, and aknown method may be adopted. Besides the active material producing stepand the anode layer forming step, the method for producing a battery inthe present disclosure may include the following steps; a cathode layerforming step of forming a cathode layer, an electrolyte layer formingstep of forming an electrode layer, and placing step of placing acathode layer, an electrode layer, and an anode layer in this order. Forthe preferable embodiment of the obtained battery, the contentsdescribed in “B. Battery” above may be appropriately referred.

Incidentally, the present disclosure is not limited to the embodiments.The embodiments are exemplification, and any other variations areintended to be included in the technical scope of the present disclosureif they have substantially the same constitution as the technical ideadescribed in the claim of the present disclosure and offer similaroperation and effect thereto.

EXAMPLES Example 1

<Synthesis of Active Material>

A simple substance of Si (purity of 99%, includes Si oxide layer on thesurface thereof), a simple substance of Al, and a simple substance of Na(purity of 99.5%) were mixed so as to be simple substance of Si:simplesubstance of Al:simple substance of Na=0.995:0.005:1.1 in the molarratio (atm ratio). In the obtained mixture, the proportion of Al to thetotal of Si and Al was 0.5 atm %. Then, the obtained mixture wasprojected into a boron nitride melting pot, the pot was sealed under anAr atmosphere, and heated under conditions of 700° C. and 20 hours so asto synthesize a Zintl compound (lumps) including Na, Si, and Al. Theobtained Zintl compound was crushed, and heated under vacuum(approximately 0.1 Pa), under conditions of 340° C. for 20 hours toremove Na, so as to obtain an intermediate. The obtained intermediatewas acid treated for 10 minutes with diluted hydrochloric acid (1N) toremove Al (by-product including Al) to obtain an active material.

<Production of Anode>

A dispersing medium (butyl butyrate), a binder (a butyl butyratesolution containing 5 weight % of dissolved polyvinylidene fluoride),the above described active material, a solid electrolyte (Li₂S—P₂S₅based glass ceramic), and a conductive material (VGCF (vapor growncarbon fiber)) were added to a polypropylene container, the containerwas shaken with a shaker (TTM-1, from Sibata Scientific Technology LTD.)for 3 minutes, and further stirred for 30 seconds with an ultrasonicdispersion apparatus to produce a slurry for an anode layer. An anodecurrent collector (Cu foil) was coated with the slurry for an anodelayer by a blade method using an applicator, then, dried for 30 minuteson a hot plate adjusted to be 100° C. An anode including an anode layerand an anode current collector was obtained in the above manner.

<Production of Cathode>

A dispersing medium (butyl butyrate), a binder (a butyl butyratesolution containing 5 weight % of dissolved polyvinylidene fluoride), acathode active material (LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ coated withlithium niobate), a solid electrolyte (Li₂S—P₂S₅ based glass ceramic),and a conductive material (VGCF (vapor grown carbon fiber)) were addedto a polypropylene container, the container was shaken with a shaker(TTM-1, from Sibata Scientific Technology LTD.) for 3 minutes, andfurther stirred for 30 seconds with an ultrasonic dispersion apparatusto produce a slurry for a cathode layer. A cathode current collector(aluminum foil) was coated with the slurry for a cathode layer by ablade method using an applicator, then, dried for 30 minutes on a hotplate adjusted to be 100° C. A cathode including a cathode layer and acathode current collector was obtained in the above manner.

<Production of Solid Electrolyte Layer>

A dispersing medium (heptane), a binder (a heptane solution containing 5weight % of dissolved butadiene rubber), and a solid electrolyte(Li₂S—P₂S₅ based glass ceramic including lithium iodide) were added to apolypropylene container, the container was stirred for 30 seconds withan ultrasonic dispersion apparatus. Then, the polypropylene containerwas shaken with a shaker for 30 minutes to produce a slurry for a solidelectrolyte layer. An aluminum foil as a substrate was coated with theslurry for a solid electrolyte layer by a blade method using anapplicator, then, dried for 30 minutes on a hot plate adjusted to be100° C. to produce a solid electrolyte layer on the substrate.

<Production of Evaluation Battery>

The anode, solid electrolyte layer, and cathode were stacked in thisorder, and the obtained stack was pressed under conditions of 130° C.,200 MPa, and 3 minutes to obtain an evaluation battery.

Examples 2 to 4

An active material and an evaluation battery were obtained in the samemanner as in Example 1 except that the proportion of Al to the total ofSi and Al (charged Al amount) was changed to 1 atm % (Example 2), 3 atm% (Example 3) and 5 atm % (Example 4).

Comparative Example 1

An active material and an evaluation battery were obtained in the samemanner as in Example 1 except that the simple substance of Al was notused.

Comparative Example 2

An active material and an evaluation battery were obtained in the samemanner as in Example 2 except that the acid treatment was not carriedout.

Comparative Example 3

An active material and an evaluation battery were obtained in the samemanner as in Example 1 except that the proportion of Al to the total ofSi and Al (charged Al amount) was changed to 10 atm %.

[Evaluation]

<Xrd Measurement>

An X-ray diffraction (XRD) measurement using a CuKα ray was conducted tothe active materials obtained in Examples 1 to 4 and ComparativeExamples 1 to 3. It was confirmed that all of the active materialsobtained in Examples 1 to 4 and Comparative Examples 1 to 3 include thesilicon clathrate II type crystal phase as a main phase. Also, thelattice constant was determined by analyzing the XRD pattern of thesilicon clathrate II type crystal phase by Rietveld method. Further,comparative values of the lattice constant in Examples 1 to 4 andComparative Examples 2 and 3 were determined by setting the latticeconstant in Comparative Example 1 as a standard. The results are shownin Table 1.

Also, as the typical results, XRD charts of the intermediate (beforeacid treatment) and the active material (after acid treatment) obtainedin Example 2 are shown in FIG. 4A and FIG. 4B. As shown in FIG. 4A, itwas confirmed that the intermediate before the acid treatment includedthe silicon clathrate II type crystal phase as a main phase, andfurther, including a peak deriving from the by-product at a position of28=34.8°, and 36.0°. Meanwhile, as shown in FIG. 4B, it was confirmedthat the active material after the acid treatment also included thesilicon clathrate II type crystal phase as a main phase as similar tothe intermediate. Meanwhile, unlike the intermediate, the peak derivingfrom the by-product was disappear in the active material after the acidtreatment. That is, it was confirmed that the by-product was removed bythe acid treatment.

<Sem-Edx Measurement>

A SEM-EDX (scanning electron microscope-energy dispersion type X-rayspectroscope) measurement was carried out for the active materialsobtained in Examples 1 to 4 and Comparative Examples 1 to 3. As thetypical result, a SEM image of the active material obtained in Example 2is shown in FIG. 5 . As shown in FIG. 5 , it was confirmed that theactive material obtained in Example 2 was porous. Also, from the EDXresult, it was confirmed that Al and Na were dispersed uniformly. Also,O was detected at the proportion higher than Al and lower than Na. Also,the proportion of Al to the total of Si and Al was determined from EDXresult, and was 0.2 atm %. Also, it was suggested by the SEM-EDXmeasurement for the intermediate that the by-product included Na, Al,and O.

For the active materials obtained in Examples 1 to 4 and ComparativeExamples 1 to 3, the cross-section of the particle was observed withSEM. The results (the results when Al was added, and acid treatment wasconducted) in Examples 1 to 4 and Comparative Example 3 are shown inFIGS. 6A to 6E. As shown in FIGS. 6A to 6E, a void was formed inside theprimary particle. Also, the void ratio was determined by image analyzingthe SEM images. The results are shown in Table 1.

<Confining Pressure Increase Measurement>

The confining pressure increase was measured by charging the evaluationbatteries obtained in Examples 1 to 4 and Comparative Examples 1 to 3.Specifically, the evaluation batteries were confined under pressure of 5MPa with a confining jig wherein a confining pressure may be measuredwith a load cell, put in a desiccator, charged at 0.1 C, to voltage of4.55 V at constant current, the confining pressure at 4.55 V wasmeasured, and the confining pressure increase from the state before thecharge was determined. The results are shown in Table 1. Incidentally,the results of the confining pressure increase in Table 1 are relativevalues when the result in Comparative Example 1 is regarded as 1.

TABLE 1 Confin- ing Lattice pressure Charged constant increase Al AcidLattice (com- Al Void (com- amount treat- constant parative contentratio parative (atm %) ment (Å) ratio) (atm %) (%) ratio) Example 0.5Treated 14.702 1.0001 0.1 5.8 0.92 1 Example 1 Treated 14.706 1.0004 0.211.7 0.86 2 Example 3 Treated 14.711 1.0007 0.5 14.9 0.79 3 Example 5Treated 14.717 1.0012 1 2.7 0.90 4 Comp. 0 Treated 14.700 1 0 0 1 Ex. 1Comp. 1 Not 14.705 1.0003 1 0 1.15 Ex. 2 treated Comp. 10 Treated 14.7081.0005 1.8 2 1.32 Ex. 3

As shown in Table 1, it was confirmed that the confining pressure wasdecreased in Examples 1 to 4, compared to Comparative Examples 1 to 3.Also, the following points were confirmed. First, in relation to thelattice constant, it was confirmed that the lattice constant were largerin Examples 1 to 4, compared to Comparative Example 1. The reason whythe lattice constant was larger is presumed because a part of Si wassubstituted with a different element Al. It is presumed that the cage ofthe silicon clathrate type crystal phase was enlarged (the density asthe active material was lowered) by this substitution, as the result, itwas possible to suppress the increase of the confining pressure evenwhen Li was inserted.

Next, in Examples 1 to 4, the by-product was removed by carrying out theacid treatment, and a void was formed inside a primary particle. Asdescribed above, in Examples 1 to 4, it is presumed that it was possibleto suppress the increase of the confining pressure even when Li wasinserted, because the void was formed inside a primary particle.Meanwhile, since Al was not added in Comparative Example 1, the void wasnot formed inside a primary particle.

Since the acid treatment was not carried out in Comparative Example 2,the void was not formed inside a primary particle. It is presumed thatsince the acid treatment was not carried out in Comparative Example 2,the by-product remained in the active material so that the effect ofsuppressing the confining pressure increase was not obtained. Also, itis presumed that the Li ion path was inhibited since the remainedby-product induced a resistance so that the resistance was increased.

Also, the confining pressure increase was larger in Comparative Example3 than in Comparative Example 1. The reason therefor is presumed that,since the charged Al amount is high, the production of the by-productwas also increased so that it was difficult to maintain the siliconclathrate type crystal phase after the acid treatment.

REFERENCE SIGNS LIST

-   -   1 . . . cathode layer    -   2 . . . anode layer    -   3 . . . electrolyte layer    -   4 . . . cathode current collector    -   5 . . . anode current collector    -   10 . . . battery

What is claimed is:
 1. An active material comprising at least Si and Al,including a silicon clathrate type crystal phase, and a proportion ofthe Al to a total of the Si and the Al is 0.1 atm % or more and 1 atm %or less and the active material is represented by Na_(x) (Si, Al)₁₃₆,where 0≤x≤20.
 2. The active material according to claim 1, wherein theactive material includes a void inside a primary particle.
 3. The activematerial according to claim 2, wherein a void ratio of the void is 2% ormore and 15% or less.
 4. The active material according to claim 1,wherein the active material includes a silicon clathrate II type crystalphase as the silicon clathrate type crystal phase.
 5. A batterycomprising a cathode layer, an anode layer, and an electrolyte layerformed between the cathode layer and the anode layer, and the anodelayer includes the active material according to claim 1.